In the CDMA system, a mobile unit needs to code and time synchronize with the serving base station before any communications with the base station can take place. In synchronous base station system, such as CDMA-2000, the same scramble code with different time offset is employed.
In asynchronous base station system, such as WCDMA, different scramble codes are used in base systems. Therefore, the mobile unit of a mobile system in WCDMA system needs not only to synchronize in the time but also to search the scramble code employed by the serving system.
Besides, the initial frequency uncertainty can be large during the synchronization. Typical voltage control temperature compensated crystal oscillators have an inaccuracy of around 10 ppm, which is equal to 20 kHz with radio frequency of 2 GHz. This frequency uncertainty induces timing inaccuracy and performance degradation. The performance of synchronization during initial cell search will cause switched-on delay of a mobile unit.
In WCDMA system, base station typically employs downlink scrambling code. There are 512 primary downlink scrambling codes for reusing. Because the method of identifying the scrambling code is complex and requires more time to synchronize with the base station, a pipelined process method in “Cell Search in W-CDMA.” (IEEE journal on Selected Areas in Communication, Vol 18, No 8, August 2000) is disclosed to accelerate the synchronization. The pipelined process reduces the time of synchronization and the average time of the spreading code synchronization. Three downlink channels are used to process code synchronization: a primary synchronization channel (P-SCH), a secondary synchronization channel (S-SCH), and a common pilot channel (CPICH). The method is carried out in three steps:
Step 1: Slot Synchronization
In step 1, the mobile unit uses the primary synchronization channel to acquire slot synchronization with a base station. This is typically done with a signal matched filter (or other similar devices) matched with the primary synchronization channel. The slot timings can be obtained by detecting the peaks of the matched filter output.
Step 2: Frame Synchronization and Code-Group Identification
In step 2, the mobile unit uses the secondary synchronization channel to achieve frame synchronization and identify the code group found in step 1. This is done by correlating the received signal with all possible secondary synchronization channel sequences and identifying the maximum correlation value. Since the cyclic shifts of the sequences are unique the code group as well as the frame synchronization is determined.
Step 3: Scrambling-Code Identification
In step 3, the mobile unit determines the exact primary scrambling channel. The primary scrambling code is typically identified through symbol-by-symbol correlation over the common pilot channel with all codes within the code group identified in step 2. After the primary scrambling code has been identified, the primary common control physical channel can be detected. The system and specific information can be read too.
The frame, 10 ms (38400 chips), can be divided into 15 time slots, for example. Each time slot can be divided into 10 symbols.
After receiving the downlink signal from the base station in the process of synchronization, the mobile unit samples the signal. According to the sample signal, the mobile unit proceeds with the slot synchronization of step 1 during a first period. During a second period, the mobile unit executes the frame synchronization and code-group identification of step 2 according to the result of step 1, and samples the signal received from the base station to execute a new step 1. During a third period, the mobile unit executes scrambling-code identification of step 3 according to the result of step 2, executes a new step 2 according to the result of the new step 1, and re-executes step 1. If the result obtained from step 3 is determined failed (the mobile unit does not synchronize with the base station), the mobile unit re-executes step 3 according to the result of the new step 2, executes the step 2 according to the result of step 1 executed during the third period, and re-executes step 1 during a fourth period, and so on. The mobile unit finishes the steps mentioned above until it synchronizes with the base station.
FIG. 1 is an example of a prior art for a mobile unit to synchronize with base station. The mobile unit receives the signal from the base station and samples the signal. First, the mobile unit executes step 1 during a first period, obtaining a first slot synchronization by using sample signal and primary synchronization channel. During a second period, the mobile unit executes step 2, obtaining a first frame synchronization and a first code-group identification according to the first slot synchronization in step 1 and the secondary synchronization channel of the sample signal. Meanwhile, in order to avoid synchronization failure, the mobile unit uses sample signal and the primary synchronization signal to re-execute step 1 and obtains a second slot synchronization during the second period.
During a third period, the mobile unit executes step 3 according to the first frame synchronization of the second period and the common pilot channel of the sample signal, and obtains a first scrambling-code identification. Meanwhile, the mobile unit executes step 2 according to the second slot synchronization obtained of the second period and the secondary synchronization channel of the sample signal, and obtains a second frame synchronization and a second code-group identification. Similarly, in order to avoid synchronization failure, the mobile unit uses sample signal and the primary synchronization signal to re-execute step 1, and obtains a third slot synchronization during the third period.
In this case, if the result of the first scrambling-code identification of the third period is determined to be that the mobile unit does not synchronize with base station, then the mobile unit executes step 3 according to the second frame synchronization, second code-group identification and common pilot channel obtained during the third period, and obtains a second scrambling-code identification. Meanwhile, the mobile unit executes step 2 according to the third slot synchronization and secondary synchronization channel, and obtains a third frame synchronization and a code-group identification. In order to avoid synchronization failure, the mobile unit uses sample signal and the primary synchronization signal to re-execute step 1, and obtains a fourth slot synchronization during the fourth period.
Since system accuracy depends on the sample rate in step 1, increasing system accuracy complicates the design of matched filter and calculating circuits, and wastes hardware resources. In addition, the inaccuracy caused by the frequency differences of timing happened during step 1 influences the accuracy of step 2 and step 3, and prolongs the synchronization time.